All species possess a circadian timing system that coordinates many physiological, biochemical and behavioral rhythms. In mammals, body temperature is an important diagnostic measurement and a commonly used marker rhythm in the study of the circadian timing system. Temperature regulation is also not constant but rather follows a daily rhythm like body temperature. For example, body temperature in a diurnally active species is elevated during the light portion of the day and depressed during the dark. The body temperature rhythm is determined by rhythms in metabolic heat production and heat loss which in turn are related to various physiological and behavioral rhythms. We are currently investigating the heat production and heat loss components of temperature regulation in the squirrel monkey. Heat production (oxygen consumption and carbon dioxide production) and heat loss (dry and evaporative) are studied using indirect calorimetry combined with a gradient layer calorimeter. Circadian rhythms, their characteristics (amplitude, phase and period), and interactions are determined under different conditions of ambient light and temperature, which influences temperature regulation. These studies are elucidating the relationship between homeostatic regulation of body temperature and the underlying circadian rhythms of heat production and heat loss. Baseline studies in ld 12:12 have been completed and published. Studies examining the relative effect of light exposure at different times of day have been completed. Studies examining the endogenous components of body temperature in constant conditions have been completed and experiments comparing the homeostatic and circadian responses to a thermal challenge are being completed.